The present invention relates to a biosensor for quantitating a specific component contained in a sample collected from a body.
Conventionally, a biosensor has been disclosed in Japanese Laid-Open Patent Publication Hei 3-202764 as a system for facilitating simple quantitation of a specific component in a sample with no need of dilution or agitation of a sample solution.
This conventional biosensor is produced by forming an electrode system having a working electrode and a counter electrode on an electrically insulating base plate by a screen printing method or the like and subsequently forming an enzyme reaction layer including a hydrophilic polymer, an oxidoreductase and an electron acceptor above the electrode system.
When the biosensor thus produced is added with a drop of a sample solution containing a substrate over the enzyme reaction layer, the enzyme reaction layer is dissolved in the sample solution. As a result, reaction between the substrate in the sample solution and the enzyme in the enzyme reaction layer will take place, which in turn causes oxidation of the substrate and reduction of the electron acceptor. Upon completion of enzyme reaction, the reduced electron acceptor is oxidized electrochemically. The concentration of the substrate in the sample solution can be quantitated based on the oxidation current value measured during this oxidizing process.
Such biosensor can quantitate various substrates if an adequate oxidoreductase fit for the measuring substrate is adopted.
Here, a glucose sensor will be described as an example of biosensor.
Conventionally known method for electrochemical quantitative measurement of glucose is a system comprising a combination of glucose oxidase (EC1.1.3.4) with an oxygen electrode or a hydrogen peroxide electrode (e.g., "Biosensor", ed. by Shuichi Suzuki, Kodansha, Japan). to D-glucono-.delta.-lactone by utilizing oxygen as an electron acceptor. In association with this oxidation reaction, the electron acceptor oxygen is reduced to hydrogen peroxide. Glucose concentration can be quantitated either by measurement of the oxygen consumption volume during this reaction using an oxygen electrode or by measurement of the produced amount of hydrogen peroxide using a hydrogen peroxide electrode.
However, this method has a drawback that the measurement is largely affected by the concentration of the oxygen contained in a sample solution, depending on the measuring material. This system has another drawback that the system can not function as a biosensor in an environment lacking oxygen.
To overcome these problems, a glucose sensor of a new type has been developed which includes an organic compound or a metal complex such as ferricyanides, ferrocene derivatives, quinone derivatives, etc. as electron acceptor, in place of oxygen.
This biosensor can carry a known amount of glucose oxidase on an electrode system, together with an electron acceptor in their stabilized state. As a result, the electrode system can be integrated with the reaction layer almost in dry state.
Such biosensor is normally disposable and facilitates measurement of the concentration of a substrate (glucose in this case) by a simple instillation of a measuring sample in a sensor chip mounted in a measurement device. Therefore, this biosensor has been attracting much attention recently and is widely applied effectively in determining diagnostic guidelines at various medical facilities.
As shown above, with this conventional biosensor, the electron acceptor present in the reaction layer is an organic compound or a metal complex such as ferricyanides, ferrocene derivatives, quinone derivatives, etc. and particularly preferred is a potassium salt.
However, the presence of potassium ions exceeding physiological level has an adverse effect on the condition of red blood cell (RBC), leading to a collapse of RBC. Such event impairs the response of the sensor to a blood sample. Therefore, it is sometimes observed that the sensor response varied depending on the number of RBCs in the blood sample, even if the substrate concentration in the blood is the same.